1. Field of the Invention
The present invention relates to the design of passive optical networks. More specifically, the present invention relates to a method and apparatus for performing bandwidth-efficient multicast in Ethernet passive optical networks.
2. Related Art
In order to keep pace with increasing Internet traffic, optical fibers and associated optical transmission equipment have been widely deployed to substantially increase the capacity of backbone networks. However, this increase in the capacity of backbone networks has not been matched by a corresponding increase in the capacity of access networks. Even with broadband solutions, such as digital subscriber line (DSL) and cable modem (CM), the limited bandwidth offered by current access networks creates a severe bottleneck in delivering high bandwidth to end users.
Among the different technologies that are presently being developed, Ethernet passive optical networks (EPONs) are a promising candidate for next-generation access networks. EPONs combine the ubiquitous Ethernet technology with inexpensive passive optics. Hence, they offer the simplicity and scalability of Ethernet with the cost-efficiency and high capacity of passive optics. In particular, due to the high bandwidth of optical fibers, EPONs are capable of accommodating broadband voice, data, and video traffic simultaneously. Such integrated service is difficult to provide with DSL or CM technology. Furthermore, EPONs are more suitable for Internet Protocol (IP) traffic, since Ethernet frames can directly encapsulate native IP packets with different sizes, whereas ATM passive optical networks (APONs) use fixed-size ATM cells and consequently require packet fragmentation and reassembly.
Typically, EPONs are used in the “first mile” of the network, which provides connectivity between the service provider's central offices and business or residential subscribers. Logically, the first mile is a point-to-multipoint network, with a central office servicing a number of subscribers. A tree topology can be used in an EPON, wherein one fiber couples the central office to a passive optical splitter, which divides and distributes downstream optical signals to subscribers and combines upstream optical signals from subscribers (see FIG. 1).
Transmissions within an EPON are typically performed between an optical line terminal (OLT) and optical networks units (ONUs) (see FIG. 2). The OLT generally resides in the central office and couples the optical access network to the metro backbone, which is typically an external network belonging to an ISP or a local exchange carrier. The ONU can be located either at the curb or at an end-user location, and can provide broadband voice, data, and video services. ONUs are typically coupled to a one by N (1×N) passive optical coupler, where N is the number of ONUs, and the passive optical coupler is typically coupled to the OLT through a single optical link. This configuration can achieve significant savings in the number of fibers and amount of hardware required by EPONs.
Communications within an EPON can be divided into upstream traffic (from ONUs to OLT) and downstream traffic (from OLT to ONUs). In the upstream direction, the ONUs need to share channel capacity and resources, because there is only one link coupling the passive optical coupler with the OLT. In the downstream direction, because of the broadcast nature of the 1×N passive optical coupler, downstream data frames are broadcast by the OLT to all ONUs and are subsequently extracted by their destination ONUs based on their individual Logic Link Identifiers (LLIDs). (An LLID carries physical address information for a frame and determines which ONU is allowed to extract the frame.)
Although assigning individual LLIDs to ONUs facilitates unicast traffic with considerable simplicity, it is not an efficient solution for performing multicast, in which the OLT needs to send the same frame to more than one ONU. With conventional unicast LLIDs, multicast frames are replicated in the OLT, and are sent downstream multiple times as unicast frames, which is very wasteful with regard to bandwidth utilization.
Hence, what is needed is a method and apparatus for providing multicast in an EPON, which is both bandwidth-efficient and cost-effective, and which meets the existing Ethernet and EPON requirements.